In reduction projection exposure devices (referred to as "steppers" hereinbelow) which use a laser to carry out the exposure processing of a circuit pattern, the amount of exposure needs to be strictly controlled in order to maintain the resolution of the circuit pattern above a certain level. On the other hand, because the excimer lasers used as light sources for such steppers are what are known as pulse-discharge excitation gas lasers, the pulse energy of every pulse varies, and there is a need to reduce this variation in order to improve the precision with which the amount of exposure is controlled.
Thus, as seen in the literature (Miyachi et al, "Excimer Laser Lithography", Kokusai Laser/Application '91, Seminar L-5, p 36-51), for example, there have been attempts to improve the precision with which the amount of exposure is controlled by means of what is known as multiple-pulse exposure wherein exposure is carried out by continuously oscillating a plurality of pulses. This is to say, this technique attempts to reduce variation in the total amount of exposure by increasing the number of exposure pulses.
Now, in a stepper, exposure and mount movement are repeated alternately. Therefore, the mode of operation of the excimer laser constituting the light source is inevitably a burst mode involving the repetition of an operation in which laser light is continuously oscillated in pulses a predetermined number of times, and then the pulse oscillation is stopped for a predetermined time. In other words, the burst mode involves the alternate repetition of a continuous pulse oscillation time and an oscillation-stopping time.
It should be noted that when "continuous pulse" and "continuous pulse oscillation" are referred to in this specification, they are used with the meaning that pulse discharge is repeatedly carried out and successive pulse laser light can be repeated; and they are used with a different meaning from "continuous oscillation laser" and "CW oscillation" as referred to more generally.
Now, as mentioned above, because excimer lasers are pulse discharge excitation gas lasers, it is difficult to always sustain the oscillation at pulse energy of a constant magnitude. Reasons for this include that the discharge produces density disturbances in the laser gas within the discharge space and causes the subsequent discharge to become non-uniform and unstable, and that such non-uniform discharges and the like produce local temperature increases at the surfaces of the discharge electrodes, and cause degradation of the subsequent discharge and cause the discharge to become non-uniform and unstable.
In particular, this tendency is marked at the start of the abovementioned continuous pulse oscillation period, and a phenomenon known as spiking occurs whereby, as shown by the portion S in FIG. 8(a), relatively high pulse energy is obtained in the initial pulses following the passage of an oscillation-stopping period, but then the discharge degrades and the pulse energy gradually reduces. This is to say, as shown in FIG. 8, even if laser oscillation is undertaken at a predetermined constant discharge voltage corresponding to a constant pulse energy Ps in order to obtain this level of energy, in practice a number of pulses of pulse energy will initially be larger than Ps.
Thus, excimer laser devices operating in burst mode have problems in that the abovementioned variation in the energy of each pulse reduces the precision with which the amount of exposure is controlled, and the phenomenon of spiking also markedly increase such variation and greatly reduces the precision with which the amount of exposure is controlled.
In recent years, moreover, the sensitivity of light-sensitive agents coated onto wafers has increased, which is contributing to increases in throughput, and therefore exposure using fewer continuous pulses has become possible and there is a tendency toward a reduced number of pulses.
However, with fewer pulses, there is a commensurate increase in the variation in the total pulse energy of the plurality of pulses, and it is more difficult to maintain the precision with which the amount of exposure is controlled using only the multiple-pulse exposure control discussed above.
In addition, in recent years, exposure methods in exposure devices for semiconductors have been moving from the stepper method in which a mount is immobilized and exposure carried out, to step & scan methods in which exposure is carried out while moving the mount. In such step & scan methods, because the exposure is carried out while moving the mount, it is not possible to use the conventional technique whereby variations in the total amount of exposure are reduced by increasing the number of exposure pulses as discussed above, and other techniques have to be used to ensure control such that the pulse energy of each pulse is uniform.
The present Applicants have applied for various patents (Japanese Patent Application No. 4-191056, Japanese Patent Application Laid-Open No. 7-106678 (Japanese Patent Application No. 5-249483) etc.) for inventions relating to what can be termed spiking-prevention control in which the initial energy rise caused by the phenomenon of spiking is prevented by varying the discharge voltage with each pulse in such a way that the discharge voltage of the initial pulse of a continuous pulse oscillation in burst mode is reduced, and the discharge voltages of the pulses are gradually increased thereafter, by making use of the property whereby the energy of the pulse oscillated increases as the discharge voltage (charge voltage) increases.
These techniques of the prior art are arranged so as to control the discharge voltage in a variable fashion in accordance with the oscillation-stopping time Tpp of the burst mode operation (see FIG. 8(a)), the charge voltage value and the like, it having been noticed that the magnitude of the spiking depends greatly on the oscillation-stopping time Tpp, the charge voltage value and the like.
However, in the prior art, on the side of the excimer laser for the stepper, laser oscillation control has been carried out using the laser oscillation synchronization signal TR, sent from the stepper side, as oscillation triggers. This is to say, as shown in FIG. 8(b), the laser side is arranged so as to receive the laser oscillation synchronization signal TR for carrying out continuous pulse oscillation sent from the stepper side, and to carry out continuous pulse oscillation synchronously with the laser oscillation synchronization signal TR which are received; only passive control being carried out on the laser side.
Therefore, on the laser side, it is not possible to know in advance what sort of burst operation mode the stepper side desires the laser oscillation to be in. This is to say, information such as the cycle of the continuous pulse oscillation, the number of continuous pulse oscillations, whether the oscillation-stopping mode is currently in effect or not, and the oscillation-stopping time is all unknown information, but the laser side has to be arranged so as to be always able to effect the correct discharge voltage control without prior warning whenever a laser oscillation synchronization signal is input.
Therefore, in the Japanese Patent Application Laid-open No. 7-106678 (Japanese Patent Application No. 5-249483), for example, the following control is carried out to enable the correct discharge voltage control to be effected whenever a laser oscillation synchronization signal TR is received.
In this technique of the prior art, charge voltage data, for matching the energy of each pulse of the continuous pulse oscillation with a predetermined target value Ps, is prestored in memory, for each pulse of the continuous pulse oscillation (what number pulse it is), using the oscillation-stopping time, the charge voltage value and the like as parameters. Further, whenever continuous pulse oscillation is carried out, the pulse energy of each pulse at that time is stored in memory, and the stored items of data are used for charge voltage control of each pulse during the subsequent continuous pulse oscillation. This is to say, each of the preceding pulse energy values which have been stored in memory is compared with the target value Ps, the abovementioned charge voltage data of each pulse, which has been stored in memory, is corrected in accordance with the results of the comparison, and charge voltage control is performed in accordance with the corrected charge voltage data.
Further, such charge voltage control is arranged such that preliminary control such as that outlined below is carried out since, as discussed above, the input timing of the laser oscillation synchronization signal TR cannot be known on the laser side. To elaborate, the laser side incorporates a timer which carries out a timing action by counting a system clock signal of a cycle shorter than the cycle of the laser oscillation synchronization signal TR, and is arranged so as to make a judgment as to whether the current system clock input instant is during the continuous oscillation or during the oscillation-stopping time, by comparing the timing output of this timer with a predetermined set time, and is arranged in such a way that, using the result of this judgment, it computes the optimum charge voltage value for each system clock input instant by carrying out the correction computation for the charge voltage discussed above at this instant, and controls the charge capacitor so as to achieve the computed charge voltage at each system clock input instant. For example, when it is judged that the current instant is during the oscillation-stopping time, the assumption is made that the interval from when a laser oscillation synchronization signal TR was last input until the current instant is the oscillation-stopping time Tpp, the charge voltage data corresponding to this assumed oscillation-stopping time Tpp is read out, a computation is carried out whereby the charge voltage data is corrected using the results of a comparison between the target value Ps for the pulse energy value and the monitor value of the first burst of energy of the preceding continuous pulse oscillation, and the charge capacitor is controlled so as to achieve the computed charge voltage.
Thus, in this technique of the prior art, the power source circuit for the discharge is always in standby m de always so as to always obtain the optimum charge value for that moment, regardless of the fact that it may be during the oscillation-stopping time, in such a way that a laser oscillation synchronization signal TR may be input at any time.
Thus, this technique of the prior art has problems in that, for example:
(a) Because the power source circuit is controlled and a high voltage is supplied, even during laser oscillation stopping operation, it is dangerous and a substantial amount of electrical energy is consumed. PA1 (b) Because the time calculation of the stopping time is carried out based on a system clock signal within the laser device, and not on the timing of when the laser oscillation synchronization signal TR is actually input, the stopping time is not accurately calculated, and the controllability of the spiking control is impaired. PA1 (c) Because the computation for the charge voltage control is constantly carried out, there is an adverse effect on the responsiveness and control speed of various other control tasks. PA1 the laser device comprises first monitor means which monitors an energy of the pulse-oscillated laser light, and first energy computation means which determines a cumulative energy or an average energy of output laser light during the continuous oscillation operation based on output of the first monitor means, and transmits the cumulative energy value or average energy to the processing device; PA1 the processing device comprises second monitor means which monitors an energy of the laser light input to the processing device, second energy computation means which determines a cumulative energy or average energy of input laser light based on output of the second monitor means, and target energy correction means which corrects a target energy value of the continuous pulse oscillation, which has been set in advance, by means of an energy value transmitted from the first energy computation means and an energy value computed by the second energy computation means, and transmits the corrected target energy value to the laser device; and PA1 the laser device further comprises control means which carries out laser output control using the received corrected target energy value as a target value.
Now, in laser exposure devices of this type, various control tasks are carried out including, on the laser device side, charge voltage control as discussed above in which the target pulse energy Ps is received from the exposure device, in such a way as to achieve the received target pulse energy Ps. At this time, on the laser device side, charge voltage control is carried out by monitoring the energy of each laser pulse actually output, and comparing the target pulse energy Ps and the monitored value, as discussed above.
However, the monitored value on the laser device side is no more than the monitored value of the energy on the laser device side, and this does not necessarily coincide with the laser energy value in the stage when the exposure is actually performed. This is to say, it sometimes happens that the monitored value of the energy on the laser device side fails to coincide with the laser energy value in the stage when the exposure is actually performed for reasons such as drift of the energy monitor of the laser device, and changes in the transmittance in the exposure device caused by mode changes in the laser beam (for example, for reasons such as part of the laser beam being kicked by a slit or the like provided in the laser-incidence aperture of the exposure device caused by widening of the laser beam).
Thus, in the prior art, the monitored value differs from the laser energy value in the stage when the exposure is actually performed, and accurate exposure control cannot therefore be carried out, since the output of a monitor placed on the laser device side is used for the monitored energy value of the laser pulse.
This invention has taken this situation into account and aims to provide a laser processing device which provides for stability in the laser device without requiring charge control during laser oscillation stopping operation, and which is able to perform high-precision laser output constancy control, and is able to improve the responsiveness and control speed of various other control tasks.
This invention further aims to provide a laser processing device which is able to perform accurate exposure control by correcting for variations in the transmittance of the laser output, and variations in the laser output monitor.